1. Field of the Invention
The present invention relates to a magnetoresistive device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each including that thin-film magnetic head.
2. Explanation of the Prior Art
In recent years, with an increase in the recording density of hard disks (HDDs), there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magnetoresistive device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
With an increase in the recording density, there has been a growing demand for the reproducing device of the reproducing head to have narrower shield gaps and narrower tracks, and there is now a GMR device with the CPP (current perpendicular to plane) structure (CPP-GMR device) proposed in the art, in which upper and lower shield layers and a magnetoresistive device are connected electrically in series to make do without any insulating layer between the shields. This technology is thought of as inevitable to achieve such recording densities as exceeding 200 Gbits/in2.
Such a CPP-GMR device has a multilayer structure comprising a first ferromagnetic layer and a second ferromagnetic layer between which an electroconductive, nonmagnetic intermediate layer is sandwiched from both its sides. A typical multilayer structure for the spin valve type CPP-GMR device comprises, in order from a substrate side, a lower electrode/antiferromagnetic layer/first ferromagnetic layer/electroconductive, nonmagnetic intermediate layer/second ferromagnetic layer/upper electrode stacked together in order.
The direction of magnetization of the first ferro-magnetic layer that is one of the ferromagnetic layers remains fixed such that when an externally applied magnetic field is zero, it is perpendicular to the direction of magnetization of the second ferromagnetic layer. The fixation of the direction of magnetization of the first ferromagnetic layer is achieved by the exchange coupling of it with an antiferromagnetic layer provided adjacent to it, whereby unidirectional anisotropic energy (also called the “exchange bias” or “coupled magnetic field”) is applied to the first ferromagnetic layer. For this reason, the first ferromagnetic layer is also called the fixed magnetization layer. By contrast, the second ferromagnetic layer is also called the free layer. Further, if the fixed magnetization layer (the first ferromagnetic layer) is configured as a triple-layer structure of a ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer (the so-called “multilayer ferri-structure” or “synthetic pinned layer”), it is then possible to give a strong exchange coupling between both ferromagnetic layers thereby effectively increasing the exchange coupling force from the antiferromagnetic layer, and to reduce influences on the free layer of a static magnetic field resulting from the fixed magnetization layer. Thus, the “synthetic pinned structure” is now in extensive use.
To meet the demands toward recent ultra-high recording densities, however, it is an essential requirement to diminish the “width” and “height” of the magnetoresistive device built in the reproducing (read) head.
To lower the height of the magnetoresistive device, viz., to make the device much thinner, U.S. Pat. Nos. 5,576,914, 6,724,583, 7,117,122, etc. have come up with a novel GMR device structure basically comprising a simple triple-layer structure of a ferromagnetic layer/nonmagnetic intermediate layer/ferromagnetic layer. According to those publications, under the action of a bias magnetic field, there is an initial state created in which the magnetizations of two magnetic layers (free layers) are inclined about 45° with respect to the track width direction. Upon detection of a signal magnetic field from a medium in the initial state of the device, the directions of magnetization of the two magnetic layers change as if scissors cut paper, with the result that there is a change in the resistance value of the device. In the present disclosure, the GMR device of such structure may be called the “scissors type GMR device” for the sake of convenience.
With the head structure using the aforesaid conventional “scissors type GMR device”, a bias magnetic field is applied to the GMR device, whereby the relative angle of magnetization of two magnetic layers (free layers) is adjusted to about 90° to create the so-called initial state. However, nowhere are the two magnetic layers (free layers) fixed, offering a problem that their magnetization directions get erratic. Further in the prior art, there is a permanent magnet provided so as to adjust the magnetization directions of two magnetic layers (free layers); however, much of the magnetic flux generated out of it leaks out toward two shield layers positioned above and below, rendering the magnetization direction adjustment function that it should have likely to decline, and making it necessary for the permanent magnet to be much larger than other parts, so ending up with much difficulty in size reductions of the whole device.
The situations being like this, the invention has been made for the purpose of a novel magnetoresistive device that, while the magnetization directions of two magnetic layers (free layers) stay stabilized, can have high reliability, and can improve linear recording densities by the adoption of a structure capable of narrowing the read gap (the gap between the upper and lower shields) thereby meeting recent demands for ultra-high recording densities, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each comprising that thin-film magnetic head.